1. Field of the Invention
The present invention relates to a finite conjugate distance zoom lens system for projection used in microfilm readers, microfilm reader printers and the like.
2. Description of the Related Art
Typically, in microfilm readers, microfilm reader printers and the like, a finite conjugate distance zoom lens is used as a projection lens to project an enlargement of a microfilm image onto a screen, and an image rotation prism is disposed on the enlarging side of said infinite conjugate distance zoom lens system between said finite conjugate distance zoom lens system and said screen, such that the image projected on the screen can be rotated by rotating said prism to correct the direction of the image. The overall length of the finite conjugate distance zoom lens system is conventionally shortened to maintain the space for disposition of the image rotation prism on the enlarging side of said finite conjugate distance zoom lens system.
A finite conjugate distance zoom lens system disclosed in Japanese Unexamined Patent Application No. HEI 4-328709, for example, consists of sequentially from the enlarging side a first lens group having negative refracting power, aperture stop, second lens group having positive refracting power, and third lens group having negative refracting power. Regarding zooming from a longest focal length state to the shortest focal length state, the first lens group is fixedly mounted relative to the image plane, and the second and third lens groups are discretely adjustable so as to move toward the reducing side, and the spacing between said second and third lens groups shifts from decreasing to increasing. The overall zoom lens system is a telephoto type due to the negative refracting power of the final lens group, i.e., the third lens group, thereby shortening the overall length of the lens.
In the aforesaid finite conjugate distance zoom lens system, the second lens group comprises sequentially from the enlarging side at least one positive lens, and a cemented lens comprising a biconvex lens and a negative lens confronting a concave surface on the enlarging side, and a positive lens. Furthermore, the third lens group comprises sequentially from the enlarging side a negative lens confronting a concave surface to the enlarging side, and a biconvex lens.
However, the finite conjugate distance zoom lens system disclosed in the aforesaid Japanese patent application has a magnification range of about 1/9.about.1/16, and a small zoom ratio of about 1.78. When the zoom ratio of the aforesaid finite conjugate distance zoom lens system is simply increased, the overall lens length is increased, and curvature of field is increased at both the maximum focal length state and minimum focal length state. Furthermore, magnification chromatic aberration is increased because the lens providing chromatic aberration correction is only a single cemented lens.